1. Field of the Invention
The invention relates to electronic screening of image signals or pixels, and more particularly, to an improved screening circuit for screening image signals or pixels.
2. Description of the Prior Art
In current electronic printing systems when an original document such as a continuous tone photograph is scanned, digitized, and printed out by a high contrast printer, it is necessary to convert the continuous tone image into a bi-level or binary image by forming halftone cells whose size varies with the intensity of the original continuous tone image. This process is called halftoning or screening. However, the topology and characteristics of the halftone cells that comprise the screen that are used to convert the continuous tone image into a bi-level image affects the appearance of the final image. Further, selection of an appropriate screen for this purpose depends on the image original, the scanner, and the printer characteristics. Because of these variables, it is desirable to have a circuit that is flexible enough to generate screens of various topologies and characteristics.
Heretofore, to generate a screen pattern, a pair of counters, one for the row or line and the other for the column or pixel, were typically used to index into a pre-programmed screen table and thereby sequentially access various screen values. In this design, for each pixel in the scan line, the pixel counter is incremented by the system pixel clock pulses to sequentially access the next location in the screen table in synchronism with each pixel in the image line. Concurrently, at the end of each scan line, the line counter is incremented to access the screen table location for the next line. However, this design approach requires that the maximum X and Y dimensions of the halftone cell be determined and fixed at the time of the circuit design. If it is desired to re-program the circuit, additional and complex hardware is required to enable the counters to be reset to a different count. Further, this approach is limited to screen applications where the halftone cell is always rectangular.
In the prior art, U.S. Pat. No. 3,900,834 to Casey et al discloses a system designed to accommodate the situation where data on a common subject is stored at different locations within a memory, with no predetermined relation between the various memory locations. When it is desired to update the data, a controller sends the update messages to a common address table containing the address of the entry to be updated, and a chain address for another entry in the table where additional data on the same subject is stored. In U.S. Pat. No. 4,419,662 to Puskas et al, a character generator is disclosed in which display data stored in a memory is addressed to a video display by means of an output latch providing latched outputs to the video display.